A photoelectrolytic cell is one in which radiant energy causes a net chemical transformation in the cell. Of particular interest are photoelectrolytic cells suitable for carrying out the photodissociation of water, forming hydrogen and oxygen at the cathode and anode respectively. Water can be photodissociated using a high energy light source (such as a laser beam) in the presence of a catalyst (such as titanium dioxide) and a separating medium which prevents the recombination of the products.
Conventional photoelectrolytic cells are typically arranged in a plane parallel configuration, with irradiation of the electrolyte occurring indirectly, i.e. the incident light passing through an electrode to reach the electrolyte. Should the photoelectrolytic reaction produce a gas, the gas will generate around an electrode, bubbling out of the electrolyte. As a result of this, the entry of additional light is impeded. This problem has been partly addressed by making the electrodes transparent and introducing any catalyst as a colloidal dispersion in the electrolyte, the aim being to reduce unwanted absorption. The presence of bubbles has been accepted as inevitable.
In a photoelectrochemical cell, a current and a voltage are simultaneously produced upon absorption of light by one or more electrodes. A specific type of photoelectrochemical cell is a photovoltaic cell, which is a solid state device, usually a semiconductor such as silicon. The device absorbs photons with energies greater than or equal to the bandgap energy, simultaneously producing electric power.
An electrolytic cell is one in which the input of electrical energy results in a net chemical transformation in the cell. A common feature of conventional electrolytic cells is that a substantial input of electrical energy is required to drive the electrolytic reaction at a sufficient rate. This expenditure of electrical energy reduces the efficiency of the cell.
Electrochemical cells, in particular electrolytic cells, may be in the form of a membrane electrode assembly (MEA). MEAs typically have a multi-layered structure comprising (i) an Ion Exchange Membrane (IEM), (ii) a current-collecting electrode, and (iii) an electro-catalyst layer on each side.
PCT/GB02/04095 describes a composite MEA formed by an in situ polymerisation process. This Application further describes an MEA having an improved reaction interface.